>Has it been possible to determine how "deep" the
biosphere was during the Mesozoic, that is, how far
under the earth's surface it "penetrated?"<
It might have been a little deeper or shallower. More likely shallower, if
geothermal grdients were steeper. These really deep microbes are just
that--microbes. They aren't higher life forms, and have probably been
around since far back in the Precambrian.
>If so, was it substantially different than the present
earth biosphere? And if so, what impact would this
have had on the animals occupying the surface?<
I would expect the "deep (subsurface) biosphere" to have had about the same
impact then as it does now. Like, we hardly know it's there.